Steam reforming methods with use of tubular reformer, e.g., so-called Topsoe method or ICI method are known as industrially available processes for producing synthesis gas or hydrogen. For instance, those kinds of processes are industrially in practical use, as being a steam reforming methods for producing reformed gas or synthesis gas, wherein a steam reforming reaction of hydrocarbons, such as methane ( CH.sub.4), is carried out in the presence of catalyst, e.g., supported nickel catalyst, so that reformed gas or synthesis gas containing a desired H.sub.2 /CO.sub.2 ratio or H.sub.2 /CO ratio of hydrogen (H.sub.2) and carbon oxide (CO.sub.2 or CO) is produced.
In a conventional steam reforming method, a combustion furnace or heating furnace is used as a reformer, which is provided with a plurality of burners. A radiant-heating section of the furnace, e.g., a vertical box type of radiant-heating section, incorporates a plurality of reformer tubes arranged substantially vertically therein, the tubes being formed by centrifugal casting alloy pipes filled with supported nickel catalyst or the like. A mixed gas of hydrocarbons and steam is preheated and introduced into the radiant-heating section to pass through the reformer tubes. The burners heat the outer wall surfaces of the reformer tubes so that the heat required for the steam reforming reaction is given to the mixed gas downwardly flowing through the tubes. Such a type of steam reforming apparatus is known as an external combustion type of reforming furnaces, for example, Topsoe type reforming furnace.
Those kinds of steam reforming methods are generally embodied as industrial system including the reforming furnace combined with another appropriate type of reformer, which is used for industrial or chemical plants for producing ammonia synthesis gas, methanol synthesis gas or hydrogen.
For instance, a chemical process for production of ammonia synthesis gas generally comprises an internal combustion reactor type of reformer (a secondary reformer) combined with the reforming furnace (a primary reformer). Primary reformed gas produced by the primary reformer is introduced into the secondary reformer together with pressurized air, and the gas is mixed with the air therein. Oxygen in the air reacts with combustible components in the primary reformed gas, whereby the heat required for reforming reaction is obtained in the secondary reformer and nitrogen gas is produced as being a secondary reformed gas to be fed to the following ammonia synthesis process.
In a case of production of methanol synthesis gas, as in the production of the ammonia synthesis gas, an appropriate internal combustion reactor type of secondary reformer is combined with the primary reforming furnace, wherein a primary reformed gas obtained by the reforming furnace is fed to the secondary reformer together with oxygen to take a reaction between the oxygen and the combustible components in the primary reformed gas, thereby ensuring the heat necessary for a chemical reaction in the secondary reformer. In this kind of system, a portion of hydrogen, which may be surplus in the following methanol synthesis process, reacts with the oxygen so as to regulate the CO/CO.sub.2 ratio to be a desired ratio proper to the methanol synthesis process.
In such systems having a combination of the primary reforming furnace and the secondary reformer, the temperature of the secondary reformed gas is set to be a relatively high temperature, and the gas produced in the secondary reformer is discharged therefrom. The gas, however, still possesses a large amount of sensible heat, which could have been effectively used in the steam reforming system. In order to effectively utilize such a sensible heat for improvement of the energy efficiency over the whole system, an improved arrangement is proposed e.g., in Japanese patent publication No.7-481, in which a heat-exchanger type of reformer having a plurality of reformer tubes in a vertical reactor vessel is used as a primary reformer.
However, it is not deemed that the steam reforming method with use of the combination of such a primary reformer and the aforementioned secondary reformer can achieve a sufficient energy efficiency. That is, it is difficult to completely supply or ensure the heat required for the reaction in the primary reformer only by the sensible heat of the secondary reformed gas effluent from the secondary reformer. Therefore, surplus air and/or oxygen has to be additionally fed to the secondary reformer so that the necessary heat of reaction in the secondary reformer is compensated for an exothermic reaction of the oxygen with the combustible components in the primary reformed gas. Such an excess feed of surplus air and/or oxygen for compensation of reaction heat is, however, involved in specific drawbacks or disadvantages. For example, if the excess air is fed to the secondary reformer in a steam reforming process for production of ammonia synthesis gas, surplus nitrogen is produced in the secondary reformer, which gas is fed to the following ammonia synthesis process. This requires an additional provision of means for removing excess nitrogen. Further, another problem is raised in that useful hydrogen in the reformed gas is considerably consumed in the secondary reformer by its reaction with the excess oxygen. If excess oxygen is fed to the secondary reformer in a steam reforming process for production of the alcohol synthesis gas or hydrogen gas, useful hydrogen in the reformed gas is considerably consumed by its reaction with the surplus oxygen. Still further, provision of compressor or blower for ensuring a pressure of reforming reaction is needed for an additional feed of the excess air or oxygen. This raises a further problem in that an additional power for driving those means is expendable, and therefore, it is not preferred for improving the energy efficiency of the whole system.
Another steam reforming method is disclosed in U.S. Pat. No. 4,162,290 in which an external combustion type of reforming furnace and a heat-exchanger type of reformer is provided in parallel so as to be used for a primary reforming step, and an internal combustion reactor type of reformer is used for a secondary reforming step, wherein an improvement of the thermal efficiency of the whole system is suggested. According to this U.S. patent, a mixed gas of hydrocarbons and steam, which is preheated in a convection-heating section of the furnace, is divided into two streams, which are introduced into the furnace and the heat-exchanger type of reformer respectively, so that the reforming reaction is partially carried out therein. The respective reformed gas streams are joined together and introduced into the internal combustion reactor type of reformer, as well as mixture of air and oxygen is introduced thereinto. Combustible components in the reformed gas of the primary reforming step reacts with the oxygen in the mixture, so that the secondary reforming step is carried out by the heat generated in the reaction, and then, the secondary reformed gas is fed to the heat-exchanger type of reformer to supply the heat of reaction therein by its heat-exchange with the mixed gas of hydrocarbons and steam.
In such a steam reforming process, it would be possible to decrease the heat for reaction in the heat-exchanger type of reformer by the parallel provision of the reforming furnace and the heat-exchanger type of reformer, whereby need for additionally feeding excess air to the internal combustion type of reformer can be restricted and effective use of the sensible heat of the secondary reformed gas can be attempted. Therefore, a thermal efficiency can be somewhat extensively improved over the whole steps.
However, in this kind of steam reforming process, the combustion exhaust gas exhausted from the reforming furnace is discharged through a convection-heating section directly connected to the furnace or additionally installed thereon, and therefore, the waste heat of the exhaust gas can be merely partially recovered. That is, the exhaust gas still has a temperature ranging from 900.degree. C. to 1,100.degree. C. This means that the heat energy of the fuel gas can be effectively used for the steam reforming reaction merely to an extent of 45%-55% of the entire exothermic heat (lower calorific value) of the fuel gas, and therefore that, the heat energy of the fuel gas cannot be effectively used for the steam reforming process.
In a plant for production of ammonia or methanol, a large mount of heat of reaction is further generated in its synthesis steps. Therefore, the total amount of sensible heat or heat of reaction, which is actually generated in the plant, extensively exceeds the total heat required for the whole plant. In general, those kinds of plants are provided with steam boilers using waste heat so as to generate steam of high pressure and high temperature (80-120 kg/cmG, 480-520.degree. C.), which is fed to the steam reforming system and a steam turbine for driving a compressor, whereby the energy efficiency is attempted to be improved in the whole plant by energy conversion of the waste heat.
However, such an indirect heat recovery of the waste heat cannot achieve the high and economical energy efficiency and production efficiency of the whole plant, and therefore, an approach of improvement is required for achieving an effective and direct use of heat energy within the steam reforming system.
It is an object of the present invention to provide a method of hydrocarbon steam reforming, which can achieve a high and economical energy efficiency and production efficiency of the whole plant.
It is another object of the present invention to provide a method of hydrocarbon steam reforming, which improves a thermal efficiency and waste heat recovery efficiency in the whole plant.